1. Field of the Invention
The invention pertains generally to computers. In particular, it pertains to navigation in digitized video and/or audio data streams.
2. Description of the Related Art
Many digitized moving picture systems use the well-known protocols and formats developed by the Moving Pictures Experts Group (MPEG), so that these protocols and formats have come to be generically referred to as MPEG. Various versions have been developed, referred to as MPEG-1, MPEG-2, etc. In an MPEG system, compressed video and audio data is packetized into elementary streams wrapped inside packet headers containing information necessary to decompress the individual streams during playback. These individual audio and video elementary streams can be further assembled, or multiplexed, into a single stream with timing information in the packet headers that identifies when in a presentation the contents of each packet should be presented. In this way, video packets can be synchronized with audio packets during playback. MPEG systems use two basic types of multiplexed streams: Program Streams (PS), which are targeted primarily for storage media, and Transport Streams (TS), which are targeted primarily for transmission and the potentially higher error rate associated with data transmission.
On the encoder side of an MPEG system, audio and video are separately compressed and packetized, and a multiplexer then combines the individual packets into a PS or TS. On the decoder side, the packets are retrieved from the stream by a demultiplexer, individual packets are depacketized and decompressed, and synchronization between audio and video is achieved by using the appropriate fields in the PS or TS headers. Decoding is typically performed on the fly as the audio/video is being played back. Because the packets are time-stamped, the playback can be manipulated to perform such functions as: moving directly to specified portions of the audio and/or video presentation, pausing, playing only audio or only video, playing audio in different languages, etc., while maintaining the proper synchronization. These and similar functions are collectively referred to as navigation. Generating navigation data for an MPEG stream is conventionally done during the encoding operation, and is placed into the MPEG stream in the form of navigation packets. Generating navigation data after the MPEG stream has been created is typically very difficult, and this function is therefore unavailable to anyone that handles the MPEG stream after encoding.